Click "Customize-Interior Modeling 2.0" and select the room to enter the modeling interface. The Extrude tool is to create a 3D solid by extruding a surface with a designated distance. Here is the instruction:
1 Select "Extrude" from the top toolbar
2 Click to select a surface.
3 Drag with the mouse to determine the extrusion distance and click again to complete the extrusion operation (you can also directly enter a value in the "Extrude-Distance" field below and hit "Enter" to finish the extrusion with the exact distance).
After selecting the surface, double click it and you could apply the last distance to extrude.
After selecting, press the "Alt" key (or ⌥ for mac) to switch between whether to keep the default extrusion surface.